Effect of calcination atmosphere on photoluminescence properties of nanocrystalline ZrO2 thin films prepared by sol–gel dip coating method

Highly transparent and homogeneous nanocrystalline ZrO₂ thin films were prepared by the sol–gel dip coating method. The X-ray diffraction (XRD) pattern of ZrO₂ thin films calcined in air, O₂ or N₂ shows the formation of tetragonal phase with varying crystallite size. X-ray photoelectron spectroscopy (XPS) gives Zr 3d and O 1s spectra of thin film annealed in air, which reveal zirconium suboxide component (ZrO_x, 0<x<2), Zr–O bond and surface defects. An average transmittance greater than 85% (in UV–vis region) is observed in all calcined samples. Photoluminescence (PL) reveals an intense emission peak at 379 nm and weak peaks at 294, 586 and 754 nm for ZrO₂ film calcined in air. An enhancement of PL intensity and red-shift is observed in films calcined in O₂ and N₂ atmosphere. This is due to the reconstruction of zirconium nanocrystal interfaces and vacancies, which help passivate the non-radiative defects. The oxygen deficient defect, which is due to the distorted Zr–O bond, is suggested to be responsible for photoluminescence. The defect states in the nanocrystalline zirconia thin films play an important role in the energy transfer process. The luminescence defects in the film make it suitable for gas sensors development and tunable lasers.     

Synthesis and characterization of nanocrystalline HAp powders prepared by using aloe vera plant extracted solution

Nanocrystalline hydroxyapatite (HAp) powders were synthesized by a simple method using aloe vera plant extracted solution. To obtain nanocrystalline HAp, the prepared precursor was calcined in air at 400–800 °C for 2 h. The phase composition of the calcined samples was studied by X-ray diffraction (XRD) technique. The XRD results confirmed the formation of HAp phase. With increasing calcination temperature, the crystallite of the HAp increased, showing the hexagonal structure of HAp with the lattice parameter, a, in a range of 0.9520–0.9536 nm and c of 0.6739–0.6928 nm. The particle sizes of the powder were obtained to be 43–171 nm. The optical properties of the calcined powders were characterized by Raman and FTIR spectroscopies. The Raman spectra showed a main peak of the phosphate vibration mode (ν₁(PO₄)) at ∼963 cm⁻¹ for all the calcined samples. The peaks of the phosphate carbonate and hydroxyl vibration modes were observed in the FTIR spectra for all the calcined powders. The morphology tends to change from a spherical shape to a rod-like shape with increasing calcination temperature as revealed by TEM.     

Synthesis and characterization of tin oxide (SnO2) nanocrystalline powders by a simple modified sol–gel route

This paper reports the synthesis and characterization of nanocrystalline tin oxide (SnO₂) powders by a simple method using a chitosan–polymer complex solution. To obtain SnO₂ nanocrystalline powders, the precursor was calcined at 500–600 °C in air for 2 h. The phase composition of calcined samples was studied by X-ray diffraction (XRD). The XRD results confirmed the formation of a SnO₂ phase with tetragonal structure. The particle sizes of the powder were found to be 22–23 nm as evaluated by the XRD line broadening method. TEM investigation revealed that the SnO₂ samples consist of crystalline particles of 19–21 nm. The corresponding selected area electron diffraction analysis further confirmed the formation of the tetragonal structure of SnO₂ without any impurity phases. The optical properties of the samples were explored by Fourier transform infrared spectroscopy, optical absorption and Raman studies. The estimated band gaps of the samples were in the range of 3.44–3.73 eV.     

Magnetic behavior of nanocrystalline powders of Co-doped ZnO diluted magnetic semiconductors synthesized by polymerizable precursor method

This paper reports the first synthesis of nanocrystalline powders of Co-doped ZnO (i.e. Zn_0.9Co_0.1O) diluted magnetic semiconductor by a polymerizable precursor method using nitrate salts of Zn and Co and a mixed solution of citric acid and ethylene glycol as a chelating agent and reaction medium, respectively. The polymeric precursors were characterized by TG-DTA to determine the thermal decomposition and crystallization temperature which was found to be at 723 K. The precursors were calcined at different temperatures of 773, 873, 973, and 1073 K for 1 h to obtain nanocrystalline powders. The morphology and crystalline size of the calcined particles were evaluated by SEM, TEM and Scherrer's equation. The average particle sizes calcined at 773, 873, 973, and 1073 K for 1 h were, respectively, 20, 60, 80, 150 nm, obtained from TEM. The XRD and Fourier transmission infrared (FT-IR) results indicated that the synthesized Zn_0.9Co_0.1O powders have the pure wurtzite structure without any significant change in the structure affected by Co substitution. Optical absorption measurements showed absorption bands indicating the presence of Co²⁺ in substitution of Zn²⁺. Room temperature magnetization results revealed a ferromagnetic behavior for the Zn_0.9Co_0.1O powders. Although the specific magnetization seemed to change with the particle size but there was no clear dependency since the largest magnetization was observed in the powders calcined at 873 K (60 nm). Instead, the specific magnetization appeared to show a trend of dependency on the lattice constant c of the wurtzite unit cell.     

The effect of La-substitution on magnetic properties of nanosized Sr1−xLaxTi0.05Zn0.2(Fe)11.75−δ(Fe)δO19 powders

A series of La-substituted M-type Sr hexaferrite powders Sr_1−xLa_xTi_0.05Zn_0.2Fe³⁺_11.75O₁₉, wherein x ranges from 0.1 to 0.5 with a step of 0.1, have been prepared by the conventional ceramic method and were then milled in a high energy mill to prepare nanosized powders. XRD investigation of the calcined and the milled powders shows that single phase hexaferrite structure has been formed after calcining and has not changed after milling. The lattice parameters and the mean crystallite sizes of the samples have been determined from the XRD data and Scherrer's formula. The results show that the lattice parameters (“а” and “c”) decrease with increase in La-substitution and the mean crystallite size of the milled powders is about 17 nm. Coercivities and magnetizations of the samples in a magnetic field of 16 kOe have been determined from the room temperature hysteresis loops. It was found that both parameters increase with La substitutions up to 0.3 and then decrease for higher substitutions. These variations were attributed to the enhancement of hyperfine field and spin-canting magnetic structure when La content increases. In addition, the magnetizations were smaller for the nanosized samples in comparison with those of bulk ones, which were discussed according to the core-shell model. Also the results show that annealing of the nanosized samples up to 500 °C can enhance coercivity and magnetization of the samples, which is discussed based on crystallite size growth.     

Study of structural and ferromagnetic properties of pure and Cd doped copper ferrite

This report presents the synthesis of copper cadmium ferrite (Cu_1−xCd_xFe₂O₄, x=0.3, 0.4, 0.5, 0.6 and 0.7) by the citrate precursor method and its subsequent characterization by X-ray diffraction (XRD), differential scanning calorimetry, infrared spectroscopy and ferromagnetic resonance. XRD results confirm the single cubic spinel phase formation with the particle size of 40 nm, which decreased up to 20 nm with increase in Cd content, while the lattice parameter increased with increase in Cd content. A significant change in the magnetic properties was observed in the CuFe₂O₄ system with Cd doping. The line width and resonance field variation against change in temperature is noted and the data is fitted to the linearlized model (LM) and Smit and Beljers (SB) model to find out the parameters. The results recorded from the SB approach are in good agreement with those observed in the magnetic measurements carried out by vibrating sample magnetometer (VSM) techniques.     

Sol–gel synthesis,magnetic and magneto-optical properties of CoFe2−xTbxO4 nanocrystalline films

The nanocrystalline thin films of terbium-doped cobalt ferrite were fabricated by a sol–gel method, and the effects of crystallization conditions on the phase, morphology, magnetic and magneto-optical (MO) properties of products were investigated. Due to its large radius, the doping content, x, of Tb³⁺ ion inside cobalt spinel cannot exceed 0.2. The CoFe_2−xTb_xO₄ films consist of the grains with the average size smaller than 50 nm even annealed up to 800°C. Saturation magnetization, coercive force and MO rotation are strongly dependent on the annealing temperature.     

Particle analysis and properties of mechanically alloyed Nd16Fe76−xTixB8

Nanocrystalline Nd₁₆Fe_76−xTi_xB₈ hard magnetic powders were prepared by mechanical alloying and respective heat treatment at 973–1073 K /30–60 min. The nanocrystalline hard magnetic powders were investigated by the NanoSight Halo LM10^TM Nanoparticle Analysis System, AFM, SEM and Mössbauer spectrometry. The nanocrystals have average size of 40 nm and the crystals form agglomerates with an average size of about 180 nm. Halo^TM, AFM and SEM techniques are the complementary methods, which give comparable results.     

Structural and magnetic studies on transition metal (Mn, Co) doped ZnO nanoparticles

We report on the structural and magnetic properties of the nanocrystalline samples of Zn_1−x(TM)_xO (TM=Mn, Co and x=0.02, 0.05, 0.10) synthesized by chemical vapor deposition (CVD) method using different carrier gases i.e., Argon (Ar), Oxygen (O₂) and Nitrogen (N₂). X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies reveal wurtzite structure of pure ZnO in all the samples and particle sizes in the range of 15-40 nm. No evidence of any secondary phases having room temperature ferromagnetic behavior has been observed through XRD and TEM studies. Magnetic measurements reveal presence of mixed magnetic phases in the samples, which may be the reason for the low saturation magnetization in the nanoparticles.     

Effects of Cd concentration on structure and optical properties of the ternary Zn1−xCdxO nanopowder prepared by sol–gel method

Zn_1−xCd_xO nanocrystalline powder with different Cd contents (0≤x≤1) has been prepared by new facile sol–gel route. The crystal structure and optical properties were investigated by X-ray diffraction patterns, Transmission electron microscope, X-ray photoelectron spectroscopy, Photoluminescence. As x varied from x=0 to 0.25, the Zn_1−xCd_xO nanopowder exhibits a hexagonal wurtzite structure of pure ZnO without any significant formation of a separated CdO phase. For the samples with 0.5≤x≤0.85, the Zn_1−xCd_xO nanopowder exhibits the coexistence of hexagonal ZnO and cubic CdO phase, meanwhile, the content of ZnO phase decreases while that of CdO increases with increasing the Cd content x. The ultra-violet near-band-edge emission of the Zn_1−xCd_xO nanopowder was monotonously red-shifted from 389 nm (x=0) to 406 nm (x=0.25) due to the direct modulation of band gap caused by Cd substitution.     

Magnetic properties of substituted strontium ferrite nanoparticles and thin films

SrFe_12−x(Zr_0.5Mg_0.5)_xO₁₉ nanoparticles and thin films with x=0-2.5 were synthesized by a sol-gel method on thermally oxidized silicon wafer (Si/SiO₂). Structural and magnetic characteristics of synthesized samples were studied employing x-rays diffraction (XRD), transmission electron microscopy (TEM), magnetic susceptometer, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). TEM micrographs display that the narrow size distribution of ferrite nanoparticles with average particle size of 50 nm were fabricated. Fitting obtained data of effective magnetic susceptibility by Vogel-Fulcher law confirms the existence of strong magnetic interaction among fine particles. XRD patterns and FE-SEM micrographs demonstrated that single phase c-axis hexagonal ferrite films with rather narrow grain size distribution were obtained. AFM micrographs exhibited that the surface roughness increases with an increase in Zr-Mg content. It was found from the VSM graphs that with an increase in substitution contents the coercivity decreases, while the saturation of magnetization increases. The Henkle plots confirms the existence of exchange coupling among nano-grain in ferrite thin films.     

Microstructural and electrical characterizations of tungsten-doped La2Mo2O9 prepared by spray pyrolysis

La₂Mo_{2 ? x} W _x O₉ (x?=?0, 0.5, 1.0, and 1.3) nanocrystalline powders were synthesized by spray pyrolysis (SP) assisted by an ultrasonic atomizer. Microstructure, sinterability and thermal stability of the prepared powders were investigated. Spherical particles of 430 to 530 nm in diameter and crystallite sizes in the 30–44 nm range were obtained. Additionally, the fine microstructure has allowed the reduction of sintering temperatures with respect to that usually reported for a solid-state reaction (SSR) method. Pellets with a variety of average grain sizes and morphologies were obtained by applying different sintering programs starting from La₂Mo_{2 ? x} W _x O₉ powders (x?=?0 and 1.3) obtained by SP and SSR processes. The electrical properties of these ionic conductors were analyzed by Electrochemical Impedance Spectroscopy (EIS) and correlated with microstructural observations. No significant variation of the electrical properties of these ionic conductors was found as compared to conventional ceramics with remarkable microstructural differences.     

Effects of calcining temperature on structural and magnetic properties of nanosized Fe-doped NiO prepared by diol-based sol−gel process

Iron-doped nickel oxide (Fe_0.01Ni_0.99O, abbreviated as FNO) nanoparticles were prepared by sol–gel process using 1,3-propanediol as a solvent and also as a chelating agent, and calcined at the various temperatures (400–1000 °C) for 2 h. The phase composition and the microstructure of the calcined products were investigated by X-ray diffraction and scanning electron microscopy techniques, respectively. Magnetic properties were measured at room temperature using a vibrating sample magnetometer. All calcined samples showed the single phase of FNO cubic rock-salt structure without the presence of any impurity phases. The crystallite size from XRD and particle size from SEM increased as calcining temperature increased. The FNO powders calcined at 400?600 °C revealed the uniform and dense spherical particles in nanosize. The room-temperature ferromagnetism was observed for all samples. When the calcining temperature was increased, the saturation magnetization decreased whereas the coercivity increased, corresponding to the less dense and larger particles. The calcined sample at 400 °C had the best magnetic properties with the highest M_s of 5.34 emu/g (at 10 kOe) and the lowest H_c of 372 Oe.     

Magnetic and microstructural properties of M-type strontium hexaferrites with Pr–ZnAl substitution

We have investigated the influence of Pr–ZnAl substitution on the magnetic and microstructural properties of M-type strontium hexaferrites Sr_1.0-xPr_xFe_12.0-x(Zn_0.5Al_0.5)_xO₁₉ (0.0 ≤ x ≤ 0.5) synthesized by the standard ceramic method. X-ray diffraction (XRD) was carried out to determine the crystal structure and the phase identification of the hexaferrites showed that a single magnetoplumbite phase was exhibited in the hexaferrites with Pr–ZnAl content (x) from 0.0 to 0.4 and impurity phase α-Fe₂O₃ was observed in the structure when Pr–ZnAl content (x)?=?0.5. The morphology of the hexaferrites was analyzed by a field emission scanning electron microscopy (FE-SEM). The representative FE-SEM micrographs showed that the particles were regular hexagonal platelets and the average grain size basically kept unchanged with increasing Pr–ZnAl content (x). A magnetic property measurement system was used to measure the magnetic properties of the hexaferrites. The remanence (B_r), maximum energy product [(BH)_max] and H_k/H_cj ratio decreased with increasing Pr–ZnAl content (x) from 0.0 to 0.5. The intrinsic coercivity (H_cj) and magnetic induction coercivity (H_cb) first increased with increasing Pr–ZnAl content (x) from 0.0 to 0.1, and decreased with increasing Pr–ZnAl content (x) from 0.1 to 0.2, and then increased when Pr–ZnAl content (x) ≥ 0.2.     

Studies of the physical properties of Co, Cu codoped ZnO powders

Zn_0.95−xCo_0.05Cu_xO powders have been synthesized by the sol-gel method and the structural, magnetic and electrical properties of the powders have been investigated. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) indicate that the Co ions do not change the ZnO wurtzite structure. Magnetic measurements indicate that Co doping can induce room temperature (RT) ferromagnetism and the addition of Cu to the powders further increases the magnetic moment per Co ion. The effects of the introduction of Cu as an acceptor dopant in the host matrix are further studied using resistance measurements. It is demonstrated experimentally that acceptor doping plays an important role in realizing dominant ferromagnetic ordering in Co doped ZnO powders.     

Formation and properties of copper-silver solid solutions upon severe deformation under pressure

Nonequilibrium nanocrystalline FCC solid solutions are obtained via the mechanical alloying of Cu_{1.0 ? x} -Ag _x powders (x = 0.1, 0.2, …, 0.9, 1.0) with deformation under pressure and their properties are investigated. The chemical homogeneity, microstructure, mechanical properties, and thermal stability of the alloys are investigated. The alloys have the positive deviation of lattice parameters from the Vegard law with crystallite sizes of 20 nm, hardness exceeding the initial values of the components by 4.5–6 times, and a brittle character of fracture. The thermally induced decomposition of nonequilibrium solutions starts at temperatures close to room and is complete after heating to 500°C with the development of collective recrystallization.     

The role of Ga substitution on magnetic and electromagnetic properties of nano-sized W-type hexagonal ferrites

A series of nano-sized single phase W-type SrGa_xZn₂Fe_16?xO₂₇ (x = 0.0, 0.1, 0.2, 0.3, 0.4) hexaferrites prepared by sol gel technique and sintered at 950 °C have been investigated. The thermal decomposition behavior of nitrate–citrate gel of as prepared powder was investigated by means of DTA/TGA analysis. The sintered powders were characterized by FTIR, XRD, SEM, VSM and vector network analyzer (VNA). X-ray diffraction patterns for pure and substituted W-type hexaferrites show the single phase structure with no impurity phase. The lattice parameters (a and c) decrease with the increase of Ga contents (x). The grain size estimated from SEM images is in the range of 139–76 nm which confirms the nanocrystalline nature of the investigated samples. The saturation magnetization (M_s) decreases whereas coercivity (H_c) increases with the increase of Ga contents (x). The values of H_c for all of the investigated samples lie in the range of few hundred oersteds which is one of the necessary conditions for EM materials. The microwave absorption property is enhanced in the frequency (with respect to ?20 dB) from 0.5 to 13 GHz, and the bandwidth reaches 0.899 GHz. The attenuation peak value is ?32 dB at the matching thickness of 3.4 mm.     

Structural and magnetic properties of sol-gel Co2xNi0.5−x Zn0.5−xFe2O4 thin films

Nanocrystalline Co_2xNi_0.5−xZn_0.5−xFe₂O₄ (x=0−0.5) thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology as well as magnetic and microwave absorption properties of the films calcined at 1073 K were studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. All films were uniform without microcracks. The Co content in the Co-Ni-Zn films resulted in a grain size ranging from 15 to 32 nm while it ranged from 33 to 49 nm in the corresponding powders. Saturation and remnant magnetization increased with increase in grain size, while coercivity demonstrated a drop due to multidomain behavior of crystallites for a given value of x. Saturation magnetization increased and remnant magnetization had a maximum as a function of grain size independent of x. In turn, coercivity increased with x independent of grain size. Complex permittivity of the Co-Ni-Zn ferrite films was measured in the frequency range 2-15 GHz. The highest hysteretic heating rate in the temperature range 315-355 K was observed in CoFe₂O₄. The maximum absorption band shifted from 13 to 11 GHz as cobalt content increased from x=0.1 to 0.2.     

Magnetic properties of BaFe11.6–2xCoxTixO19 particles produced by sol–gel and spray-drying

Ba-ferrite powders of composition BaFe_11.6−2xCo_xTi_xO₁₉ (with x varying from 0 to 1.1) were prepared by the sol–gel and spray-drying techniques. Green powders showed an amorphous structure. They were heat-treated from 850°C to 1000°C to promote crystallisation and to study the dependence of the magnetic properties on the annealing temperature. Unlike the evaporation-dried powders, the spray-dried samples showed a highly homogeneous structure, which consisted of spherical particles with sizes ranging from 0.1 to ∼0.3 μm. As for the magnetic properties, the saturation magnetisation and remanence remained practically constant up to x=0.8 for all heating temperatures, whereas the coercivity fell uniformly as x increased from ∼5.5 to ∼0.1 kOe.     

An investigation on microstructural,spectral and magnetic properties of Pr–Cu double-substituted M-type Ba–Sr hexaferrites

This is first report on Pr–Cu double-substituted M-type Ba–Sr hexaferrites with nominal compositions Ba_0.3Sr_0.7−xPr_xFe_12.0−xCu_xO₁₉ (x = 0.00–0.35) fabricated by the solid-state reaction route. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM) and Hysteresis graph meter were used to characterize the synthesized M-type Ba–Sr hexaferrites. XRD results show that the hexaferrites with x ≤ 0.21 exhibited single M-type phase, while the hexaferrites with x ≥ 0.28 exhibited the M-type phase and impurity phases. FE-SEM images proposed that all particles with hexagonal platelet-like shape were homogeneously dispersed. The remanence (B_r) first increased with Pr–Cu content (x) from 0.00 to 0.14, and then decreased when x ≥ 0.14. The intrinsic coercivity (H_cj) and magnetic induction coercivity (H_cb) first decreased with x from 0.00 to 0.14, and then increased when x ≥ 0.14. Maximum energy product [(BH)_max] reached the maximum value at x = 0.14.